Raise warning or error if simulated physics or numerics is questionable

Such cases include

• incident grazing angle too close to zero (behavior changed in e9742e49, effect rediscovered in #1163 (closed))
• strong form factor oscillations at small qR

Subsequently, adequate display must be provided in the GUI.

At this occasion, revise the result data type, and possibly make it more pythonic (Redmine2469).

Transferred from https://github.com/scgmlz/BornAgain/issues/1220. Redmine2479 (unify GUI warnings) shall also be subsumed here.